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# On/off chillers EnergyPlus

I'm modelling a chilled water loop with EnergyPlus. I used WaterUse:Equipment to model the chilled water demand and a Chiller:Electric object on the plant side. The project consists in the evaluation of various chiller replacement options for this application. Among the different options, I need to model chillers with and without inverter (i.e. variable capacity and on/off operation). How can I model this difference in operation with EnergyPlus?

EDIT:

I modelled the on/of chillers, as suggested by Julien, setting the minimum PLR and minimum unloading ratio to 1. Then I tried to compare it with a continuously regulated chiller. Below you can see the same chiller (same nominal power, COP, mass flow rate, performance curves) operated in the on/off mode (blue) and continuously (minimum PLR and Unloading ratio set to 0.2, red). I came up with the odd result that the on/off chiller is more convenient than the inverter regulated chiller. It seems to me that the cycling ratio is somehow equivalent to the PLR, in terms of how it scales down the available capacity to the actual demand, neglecting that, in fact, inverter regulation of the compressor motors allows to smoothly follow the load, avoiding peak startup current.

Did I interpret it correctly? Is there any way to make the model more consistent, taking into account the benefits of inverter regulation? Thanks

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Make sure you report at "detailed" frequency, not "timestep".

( 2018-01-29 03:01:26 -0500 )edit

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Don't use a WaterUse:Equipment for this (this is something to model (typically hot) water usage at a faucet, all water used is replaced by mains water), but use a LoadProfile:Plant instead.

As far as on/off chillers, there aren't any specific objects to do this, in particular for "modular" type chillers that have several compressors. You could however use a Chiller:Electric:EIR and force the Minimum Part Load Ratio to something close to 1 to force your chiller to operate at rated capacity (haven't tested myself). Make sure you set the Minimum Unloading Ratio to the same value as the Min PLR.

If your chiller is of modular type, say with 3 identical compressors, you'd model three identical chillers corresponding to each compressor.

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Thank you! Indeed I had to force mains water temperature with a schedule to close the cycle, but with LoadProfile it's much easier.

( 2018-01-24 10:58:47 -0500 )edit